I think the only champion UFC would privately tell you they want to replace is Tim Sylvia, since a main event with just him can't sell a PPV since he's not that exciting a fighter or personality.
What UFC is doing is increasing the level of competition. This small pond is betting bigger, so these fish that ruled when the pond was small, are going to find out just how good they really are when UFC starts letting some big fish in.
BTW, I thought the champions deserved immediate rematches too. However, moneywise UFC wants to build their champions back up as it'll make for a bigger anticipated rematch in the future.
2007 is going to to be awesome for UFC with Hughes/GSP rematch, Frankin/Silva again, Crocop/Syvlia, and Rampage/Chuck.